1. Field of the Invention
The invention generally relates to the field of measuring while drilling of earth boreholes and, more particularly, to the determination, during a drilling operation with the drill string in a fluid-containing borehole, of virgin formation pressure of formations surrounding the borehole.
2. Background of the Invention
Existing well logging devices can provide useful information about hydraulic properties of formations, such as pressures and fluid flow rates, and can obtain formation fluid samples for uphole analysis. Reference can be made, for example, to U.S. Pat. Nos. 3,934,468 and 4,860,581. In a logging device of this general type, a setting arm or setting pistons can be used to controllably urge the body of the logging device against a side of the borehole at a selected depth. The side of the device that is urged against the borehole wall includes a packer which surrounds a probe. As the setting arm extends, the probe is inserted into the formation, and the packer then sets the probe in position and forms a seal around the probe, whereupon formation pressure can be measured and fluids can be withdrawn from the formation. The probe typically penetrates the mudcake and communicates with the formation adjacent the mudcake by abutting or slightly penetrating the formations. The pressure measured with the probe at the formation adjacent to the mudcake is sometimes called the “probe pressure” and it can be used as an indicator of the virgin formation pressure, it being understood that there will often be substantial invasion of the formations near the probe. However, the measurement of true formation pressure, especially in relatively low permeability formations, is sometimes rendered difficult or impossible by a phenomenon called “supercharging”.
According to one theory, supercharging is caused by the fact that the permeability of mudcake is not exactly zero, but has some small finite value. In low permeability formations, the resistance to fluid flow due to the mudcake can be of the same order of magnitude as the resistance of the formation to accepting the fluid. Thus, a standard wireline pressure measurement, which measures the pressure difference across the mudcake, will not be sufficient to measure the pressure of virgin formation, since there remains (due to the constant fluid flow across the mudcake), a residual finite pressure difference between the formation at the mudcake interface and virgin formation far away.
As described in applicant's U.S. Pat. No. 5,798,669, an explanation of supercharging can be made by analogy to electrical current flow, since Darcy's law and Ohm's law have the same algebraic form. Reference can be made to the diagram of FIG. 1. The pressure difference between the borehole (hydrostatic) and the virgin formation is the driving potential Vbh−Vf. The mudcake is analogous to a relatively high value resistor Rmc. The formation is another resistor, Rf, in series with the mudcake. A high permeability formation is represented by a low formation resistor. In such a case Rmc>>Rf, and the whole potential drop will occur across the mudcake resistor, and a potential measurement across the mudcake Vbh−Vmc will provide the formation potential, as Vmc=Vf. For impermeable formations Rmc<<Rf, and there will be almost no potential difference observed across the mudcake, so Vmc=Vbh.
However, for low permeability formations, where Rmc and Rf are of the same order of magnitude, Vmc will be somewhere between Vbh and Vf. Since Vmc is the analog of the probe pressure measurement taken with the above-described type of logging tool, it is seen that in this case the true reservoir pressure will not be obtained by having the measurements Vbh and Vmc.
As noted in the '669 Patent, instead of making a single probe pressure measurement at a point in the well, the well hydrostatic pressure can be used as the driving potential, and additional probe pressure measurements can be made with different driving potentials. From two such measurements, when the difference in the driving pressures is of the same order of magnitude as the difference between the driving pressure and the formation pressure, the formation pressure can be determined. The technique can be extended to several measurements, to improve the precision of the result.
As described in an embodiment in the '669 Patent, there is provided a method for determining true formation pressure in formations surrounding a fluid-containing borehole having a mudcake on the surface thereof, including the following steps: with the pressure in the borehole at a first measured borehole pressure, measuring, as a first probe pressure, the pressure in the formation adjacent the mudcake; with the pressure in the borehole at a second measured borehole pressure, measuring, as a second probe pressure, the pressure in the formation adjacent the mudcake; and deriving the true formation pressure from the first and second measured borehole pressures and the first and second probe pressures.
As further described in the '669 Patent. In situations where the borehole hydrostatic pressure will naturally vary over a short period of time (for example, in certain floating rig situations), it may not be necessary to vary the hydrostatic pressure. In such cases, the readings of hydrostatic pressure as a function of time can show pressure variations, and if they are significant, the pressures measured with the probe can be utilized, in conjunction with the hydrostatic borehole pressure measurements. In other situations, the borehole hydrostatic pressure can be varied in other suitable ways, for example, increasing or decreasing pressure by pumping or by removal of fluid, although it is noted that lowering of pressure, in some circumstances, would not be recommended from a safety standpoint.
As further described in the '669 Patent, borehole pressure variation can also be localized to the region in which measurements are being made, using dual packers. The pressure within the isolated region of the borehole can be modified by pumping to or from (preferably to) the isolated region. As shown in the '669 Patent, this is implemented by providing packers and a pump-out module as part of the apparatus used to perform the pressure measurements.
It would be advantageous to have a technique and apparatus that is capable of efficiently determining, while drilling, with the drill string in the borehole, the true formation pressure, even under conditions where supercharging is occurring, and it is among the objectives of the invention to provide this capability.